1. Field of the Invention
This invention relates to aluminum alloy shapes and a method of making the same, and more particularly to aluminium alloy shapes whose aging is ensured to properly proceed by achieving heat treatment at a temperature below 200.degree. C. for a short period of time and a method of making such aluminum alloy shapes.
2. Description of the Prior Art
An age hardening aluminum alloy has recently been developed, whose mechanical properties compare favorably with those of steel or like materials and which is light-weight, highly anti-corrosive and small in deformation resistance. Accordingly, it is used for various purposes, in particular, widely used for construction materials. Those aluminum alloy shapes now used for construction materials are placed on the market after coated with paint.
A conventional method of making aluminum alloy shapes is as follows.
As illustrated in FIG. 1, the manufacture starts with homogenization treatment of a cast ingot of aluminum alloy, for example, at 550.degree. C. for 2 to 3 hours. The cast ingot is then pre-heated, for example, at 400.degree. to 500.degree. C. for 5 to 10 minutes and formed by extrusion in a predetermined shape.
Next, the extruded shapes of the predetermined shape thus obtained are heated at 205.degree. C. .+-. 5.degree. C. for 60 minutes to cause aging to proceed. Thereafter, the extruded shapes are subjected to ground film forming, coating, coated film printing and hardening and like treatments to provide aluminum alloy shapes.
In the conventional manufacturing method, however, substantially no consideration is paid to economy of energy and simplification of the manufacturing processes, so that there are many problems to be solved. To overcome such problems, the present inventors devised such a method as shown in FIG. 2 in which the extruded shapes are immediately subjected to the ground film forming and coating treatments without artificially expediting aging of the aluminum alloy and then heat-treated to thereby effect printing and hardening of the coated film and, also, age hardening.
With this manufacturing method, the ground film forming, coating and other treatments are achieved before age hardening of the aluminum alloy, so that these treatments can be easily performed and all the processes from extrusion forming to coating can be designed on a continuous system. Further, since aging is artificially caused to proceed simultaneously with the coated film printing and hardening process, heat treatment for artificial aging can be saved, which accomplishes an economy of energy and, also, ensures close contact of the coated film with the shapes.
However, the manufacturing method shown in FIG. 2 makes it necessary that the condition for aging of the aluminum alloy and that for coated film printing and hardening are substantially coincident with each other. It is very difficult to satisfy this requirement on an industrial scale.
Namely, an aluminum alloy commercially known under the name of A.A6063 is most widely used for construction materials. This aluminum alloy is a typical age hardening alloy and a highly excellent alloy such that when it is in the state of a cast ingot, a required extrusion property is satisfied by homogenization treatment and preheat treatment and that aging is artificially caused to proceed by subsequent heating to provide mechanical characteristics. Further, this alloy is defined to contain 0.52% of magnesium and 0.45% of silicon, and the alloy now on the market contains such materials exactly or substantially in the defined amounts. In the case of this alloy, the extrusion property is not impaired and when it is heat-treated at 205.degree. C. .+-. 5.degree. C. for 60 minutes, aging properly proceeds to provide predetermined mechanical properties. However, if the conditions for aging are altered, that is, if the time for aging is shortened and if the aging temperature is lowered, the predetermined mechanical properties can not be obtained.
Accordingly, in the case where the coated film printing and hardening condition and the artificial aging condition are made coincident with each other in this alloy on the market, if no special paint is used, it is required to lower the heating temperature and unnecessarily lengthen the heat treatment time so as not to deteriorate properties of the coated film. However, this brings about unfavorable results.
On the other hand, it is considered possible that if a special paint fit with the aging conditions of the alloy on the market is employed, the coated film printing and hardening and the age hardening of the alloy are achieved at the same time. However, it is technically difficult to raise only the printing and hardening temperature, for example, up to 205.degree. C. .+-. 5.degree. C. without impairing the water solubility of the paint which is the most suitable for dip coating. Even if this problem is technically solved, the special paint contains an expensive composition, and hence is very costly.
Further, considering the aging conditions of the alloy on the market from the viewpoint of energy, the aging temperature of 205.degree. C. .+-. 5.degree. C. is too high and it is desired to lower the temperature and the aging time of 60 minutes is also too long and it is preferred to shorten this time.